ul{
    list-style: none;
    padding: 0;
    margin: 0;
}
.nav{
    width: 800px;
    margin: 0 auto;
    height: 40px;
    background: #ccc;
}
.nav>li{
    float: left;
    height: 40px;
    line-height: 40px;
    width: 200px;
    text-align: center;
}
.nav>li:hover ul{
    display: block;
}
.nav>li>ul{
    display: none;
}